The present invention relates to a piezoelectric ceramic material, a method of manufacturing the piezoelectric ceramic material, and to an electroceramic multilayer component having such a piezoelectric ceramic material.
Piezoelectric ceramics which are used in actuators, for example, are often manufactured using lead zirconate titanate (PZT) mixed crystal. If suitable additives or dopants are used, these piezoelectric ceramics have advantageous property combinations such as high temperature resistance, high piezoelectric charge constant, high Curie temperature, low dielectric constant, and low coercive field intensity. It is, however, disadvantageous that some of the maximally achievable and desired properties of these PZT ceramics are not obtained when conventional additives are used.
Considering a PZT structure as an A2+B4+O2xe2x88x923 structure, ceramics which undergo A or B site substitution by a higher-valency cation (donor) are designated xe2x80x9csoft ceramics.xe2x80x9d This substitution produces lead vacancies, so that such ceramics have primarily a high piezoelectric charge constant, high dielectric constant, high dielectric and mechanical loss, low coercive field intensity, and easy polarizability. Typical additives producing these properties include, for example, La3+ or, in general, oxides of the rare-earth elements or also Bi3+ for A site substitution. Ta5+, Nb5+, W6+, or Sb5+ may be considered for B site substitution.
On the other hand, PZT ceramics which undergo A or B site substitution by a lower valency cation (acceptor) are, designated xe2x80x9chard ceramics.xe2x80x9d In this case, oxygen vacancies are produced, so that such ceramics have a low piezoelectric charge constant, low dielectric and mechanic loss, high coercive field intensity, and low electrical resistance. Furthermore, such ceramics are usually difficult to polarize. Typical additives inducing such properties in-such xe2x80x9chard ceramicsxe2x80x9d are, for example, K1+ or Na1+ for A site substitution. Primarily Ni2+, Zn2+, Co2+, Fe3+, Sc3+, or Mg2+ may be considered for B site substitution.
In summary, primarily materials having a combination of the properties of xe2x80x9csoft ceramicsxe2x80x9d and xe2x80x9chard ceramicsxe2x80x9d are relevant for use in a piezoelectric actuator as PZT ceramics. In particular, it is advantageous if the piezoelectric charge constant and the Curie temperature are as high as possible, i.e., d33 greater than 500 1012 m/V and Tc greater than 300xc2x0 C., and if the dielectric constant of the material obtained is as low as possible, i.e., if ∈33/∈0 is less than 2000.
In order to meet these requirements, it has been proposed that the PZT ceramic be codoped, resulting in the formation of both Pb vacancies and oxygen vacancies. Thus, German Patent Application No. 196 15 695 proposes that the surface of pure donor-doped PZT green ceramics be provided with an Agx/Pd1xe2x88x92x paste (x=0.7) and that these ceramics be stacked, with silver diffusing into the adjacent ceramic layers in a subsequent joint sintering (cofiring) of the green ceramic which has the Ag/Pd paste and being built into an A site as an acceptor. Furthermore, European Patent No. 0 619 279 proposes that doping be performed using complex compounds having the general formula A(B11xe2x88x92xB2x) where A=Pb and B1=monovalent, bivalent, or trivalent cations, and B2=trivalent, pentavalent, or hexavalent cations, i.e., compounds of the type A(W1/3Ni2/3)O3 or A(Mg1/3Nb2/3)O3, for example, where A may be lead, strontium, calcium, or barium.
Additionally, Published PCT International Application No. 99/12865 proposes doping a PZT ceramic with lead-free complex compounds which have a Perowski structure like the PZT mixed crystal and which react to yield a single-phase mixed crystal when added in small amounts and jointly calcined. Such compounds have the general composition A2+B10.25+B20.755+O3 where A=barium and/or strontium, B1=potassium and/or sodium, and B2=niobium, tantalum, or antimony. The disclosed required sintering temperatures for producing an electroceramic multilayer component having such a piezoelectric ceramic material are below 1150xc2x0 C.
An object of the present invention is to provide a piezoelectric ceramic material having the highest possible thermal stability, piezoelectric charge constant, and Curie temperature, and the lowest possible dielectric constant, electromechanical losses, and also a low coercive field intensity, and a low electrical conductivity. Furthermore, another object is to provide a material sinterable at temperatures below 1000xc2x0 C. so that by using this material a more cost-effective internal electrode paste could be used in the manufacture of electroceramic multilayer components.
The piezoelectric ceramic material according to an embodiment of the present invention has the advantage over the related art in that it has a high thermal stability, a piezoelectric charge constant d33 greater than 500 1012 m/V, a Curie temperature Tc higher than 300xc2x0 C., a dielectric constant ∈33/∈0 less than 2000, and a coercive field intensity less than 1.2 kV/mm with low electromechanical losses, and low electrical conductivity. Furthermore, this piezoelectric ceramic material is sinterable, for example, together with copper-based or AgtPd1xe2x88x92t-based electrode paste layers (where txe2x89xa70.7 and t is the proportion by weight) at temperatures below 1000xc2x0 C. in a cofiring process to yield an electroceramic multilayer component. The sintering may advantageously take place in the presence of an air, nitrogen, or a nitrogen-containing gas atmosphere.
According to a particular embodiment, this lower sintering temperature allows the use of a silver-palladium mixture as the material for the electrode paste layers, whose Ag content is considerably greater than 70% by weight and is less expensive than platinum-based internal electrode materials, for example, or internal electrodes having a higher palladium content.
The use of precious metal-free electrode paste layers, i.e., precious metal-free pastes such as copper pastes, for example, offers considerable cost advantages. Additionally, an undesirable diffusion of copper from the internal electrode layers into the adjacent ceramic material can be avoided at the same time by adding copper oxide to the piezoelectric ceramic material as an additive or dopant. In this case, the PZT ceramic is already saturated with copper ions due to the copper oxide addition to the PZT ceramic. In addition, the possibility of using copper as an alternative to silver/palladium limits the dependence of the price of the resulting electroceramic multilayer components on the highly speculative prices of palladium and platinum.
In a method of manufacturing the piezoelectric ceramic material according to the present invention, it is advantageous that all additives and dopants used in addition to lead, zirconium, and titanium may be used as powdered oxides or carbonates, which are available at a reasonable cost and in large quantities.
Thus, it is advantageous to add copper in the form of a Cu1+ and/or Cu2+ ion as a dapant to the piezoelectric ceramic material, which allows sintering of the green ceramic, to which this material is added as a ceramic component together with a copper-containing electrode paste layer, in the presence of an air, nitrogen, or a nitrogen-containing atmosphere possible.
The piezoelectric material obtained may also have either a stoichiometric composition or a composition containing a lead oxide, in particular PbO, in a stoichiometric excess, which causes a further reduction in the sintering temperature.
In addition, a castable slurry or an extrudable compound for the manufacture of green ceramics can be produced conveniently as is known by those skilled in the art.